Identification of Inhibitors and Substrates of Phex (2004-07)

The aim of this research is to develop molecular probes using a phage display technique that will recognize Phex, and identify its substrates and inhibitors. Phex is the PHosphate-regulating gene homologous to Endopeptidases on the X chromosome. Phex is the disease-causing gene in XLH, an inherited form of rickets, a disorder involving softening and weakening of the bones.

X-Linked Hypophosphatemia (XLH) is
the most common inherited form of
rickets. XLH is caused by inactive mutations
of Phex. Hyp mouse is a disease model of
XLH. The Hyp phenotype can be transferred
from a hyp to a normal mouse by surgically
linking their circulation. Kidney transplant
between normal and Hyp mice shows that
XLH is not a primary kidney defect. Phex is
mainly expressed in bone on the cell surface
of osteoblasts.

Phage display is a relatively new in vitro screening technique that is used to rapidly sift through a library of proteins or peptides. Vast libraries of specialized peptides, antibodies and cDNA can be screened efficiently for binding to a variety of target molecules by incubating the phage library with the target. Non-binding phage is washed away, while the remaining phages are amplified. After several rounds of screening, individual clones are characterized by ELISA and DNA sequencing.

Dr. Liu's plan is to screen an antibody library to determine which antibodies inhibit Phex function; these identified antibodies can then serve as a function blocker for Phex. He will also identify Phex substrates by screening a cDNA substrate library.

Validated Phex antibodies have strong potential for development as inhibitors of Phex function, and as biomarker-specific probes with applications in basic science, therapeutics, diagnostics, and imaging. Identification of Phex substrates also will be important in identifying the intermediate steps linking Phex to FGF23-mediated phosphaturia, an excessive discharge of phosphates in the urine.